1. Field of the Invention
The present invention relates to a magneto-optical recording-reproducing method comprising projecting a light beam on a magneto-optical medium of a multi-layer structure, displacing a domain wall of a record mark in a displacement layer without change of recorded data in a memory layer by utilizing temperature gradient in temperature distribution, and detecting a change of polarization direction of the reflected light beam to reproduce the record mark of less than light diffraction limit. The present invention relates also to a magneto-optical recording-reproducing apparatus employing the above method.
2. Related Background Art
Magneto-optical mediums for rewritable high-density recording are known which record information by forming magnetic domains in a magnetic thin film by utilizing thermal energy of a semiconductor laser and read out the information by utilizing a magneto-optical effect. In recent years, higher recording density of the magneto-optical medium is demanded. In an optical disk such as magneto-optical medium, the linear recording density depends greatly on the laser wavelength and the numerical aperture of the objective lens of the reproducing optical system. More specifically, the laser wavelength .lambda. and the numerical aperture NA of the objective lens of the reproducing optical system decide the diameter of the beam waist, whereby the detectable range of the spatial frequency of record mark reproduction is limited to about 2NA/.lambda.. Therefore, for achieving higher recording density with a conventional optical disk, the laser wave length should be shorter and the NA of the objective lens should be larger in the reproducing optical system. However, the improvements in the laser wavelength and the numerical aperture of the objective lens is limited naturally.
For further higher recording density, the constitution of the recording medium and the reading method are being improved. For example, Japanese Patent Laid-Open No. 06-290496 discloses a signal-reproducing method and an apparatus therefor, in which signals are recorded in a multi-layered film having a displacement layer and a record-storing layer coupled magnetically, and the record marks of less than the light diffraction limit is reproduced by displacing the domain wall of record marks in the displacement layer by utilizing a temperature gradient caused by irradiation of heating light beam without changing recorded data in the record-storing layer, magnetizing uniformly and almost entirely the light beam spot region on the displacement layer, detecting the change of polarization direction of the reflected light beam. This method reproduces signals in a rectangle form as shown in FIG. 2E, which enables reproduction of record marks of frequency of less than optical diffraction limit without decreasing the reproduction signal amplitude. Thereby, the medium and method for the magneto-optical recording are greatly improved in the recording density and the transfer speed.
FIG. 1 shows a constitution of a conventional system. In FIG. 1, magneto-optical disk 1 is constituted of substrate 2, magneto-optical layer 3 formed thereon, and protection layer 4 formed further thereon. Substrate 2 is formed from glass or a plastic material. Magneto-optical layer 3 is capable of reproducing record marks of less than optical diffraction limit by shifting a domain wall by utilizing temperature gradient caused by light beam irradiation without changing recorded data in the record-storing layer, magnetizing uniformly and almost entirely the light beam spot region on the displacement layer, and detecting the change of polarization direction of the reflected light beam. Magneto-optical disk 1 is fixed to a spindle motor by a magnet chucking or a like means to be rotatable on a rotation axis.
Parts 5 to 17 constitute an optical head for projecting a laser beam to magneto-optical disk 1 and for receiving information from reflected light. The parts comprise condenser lens 6 as an objective lens, actuator 5 for driving condenser lens 6, semiconductor laser 7 of a wavelength of 680 nm for record reproduction, semiconductor laser 8 of wavelength of 1.3 .mu.m for heating, collimator lenses 9,10, dichroic mirror 11 for completely transmitting light of 680 nm and completely reflecting light of 1.3 .mu.m, beam splitter 12, dichroic mirror 13 for intercepting light of 1.3 .mu.m and completely transmitting light of 680 nm to prevent leakage of light of 1.3 .mu.m into the signal detecting system, .lambda./2 plate 14, polarized light beam splitter 15 for splitting the laser beam according to polarization angles into two directions, photosensors 17, condenser lenses 16 for photosensor, and differential amplification circuit 18 for differentially amplifying the condensed and detected signals for respective polarization direction.
The laser beams of 680 nm and 1.3 .mu.m emitted respectively from semiconductor lasers 7,8 for recording-reproducing and heating are introduced through collimator lenses 9,10, dichroic mirror 11, beam splitter 12, and condenser lens 6 to magneto-optical disk 1. Condenser lens 6 moves in the focusing direction and the tracking direction under control by actuator 5 to focus the laser beams successively on magneto-optical layer 3 by tracking along a guiding groove formed on magneto-optical disk 1. The light flux of 1.3 .mu.m is made smaller than the aperture diameter of condenser lens 6 to make the NA smaller than that of the light of 680 nm which is condensed through the entire area of the aperture.
The heating spot, which is formed with a larger wavelength and a smaller NA, has a larger diameter of heating beam 74 than the recording-reproducing spot of recording-reproducing beam 73 as shown in FIGS. 3A and 3B. Ts isotherm is indicated by numeral 75. Thereby, a desired temperature gradient is formed in the recording-reproducing spot region on the moving disk face as shown in FIG. 3D. The laser beam reflected by magneto-optical disk 1 is deflected by beam splitter 12 to the optical path toward polarized light beam splitter 15, and travels through dichroic mirror 13, .lambda./2 plate 14, and polarized light beam splitter 15. The split light beams are respectively condensed by lenses 16 onto sensors 17 corresponding to magnetization polarity of the spot on magneto-optical layer 3. The condensed light beams are composed only of 680 nm light since dichroic mirror 13 intercepts the 1.3 .mu.m light. The outputs from the respective photosensors 17 are amplified differentially by differential amplifier 18 to output the magneto-optical signals from terminal 90. Controller 20 receives information on rotation rate of magneto-optical disk 1, recording radius, recording sectors, and so forth and outputs recording power for LD (Laser Diode) power setting, and recording signals to control LD driver 19, and magnetic head driver 24. LD driver 19 drives semiconductor lasers 7,8. In this example, LD driver applies recording power and reproducing power to semiconductor laser 7, and heating beam power to semiconductor laser 8 to control them.
Magnetic head 23 applies a modulation magnetic field onto the laser irradiation site on magneto-optical disk 1 for the recording operation. Magnetic head 23 is placed in opposition to condenser lens 6 with interposition of magneto-optical disk 1. During recording, recording-reproducing semiconductor laser 7 applies recording laser power by DC (Direct Current) light irradiation, and synchronously magnetic head 23 produces magnetic fields of different polarities under control by magnetic head driver 24 in correspondence with the recording signals. Magnetic head 23 moves with the optical head in a radius direction of magneto-optical disk, and applies a magnetic field successively on recording onto the laser irradiation site of magneto-optical layer 3. Magneto-optical layer 3 is constituted of three layers of a displacement layer, a switching layer, and a memory layer, each having a domain wall structure shown by arrow marks.
The recording-reproducing operation is explained by reference to FIGS. 2A to 2F. FIG. 2A shows recording signals, FIG. 2B a recording power, FIG. 2C modulating magnetic fields, FIG. 2D record marks, FIG. 2E reproducing signals, and FIG. 2F binary digit signals. In recording of the recording signals as shown in FIG. 2A, the power of semiconductor laser 7 is controlled to be at a prescribed level from the start of the recording operation, and modulating magnetic field is applied in accordance with the recording signals. Thereby, record mark sequence is formed in the process of cooling of memory layer as shown in FIG. 2D, where the line-shadowed portions show magnetic domains magnetized in the direction corresponding to the record marks in the present invention, and the dot-shadowed portions show magnetic domains magnetized in the reverse direction.
The reproduction operation is explained below by reference to FIGS. 3A to 3C. Numeral 99 indicates groove, and numeral 100 indicates land. The medium is heated up to a temperature for causing the displacement of the domain wall in the displacement layer of the medium by a heating beam 74. The isotherm 75 of the temperature Ts of the recording medium, which is the main factor for inducing displacement of the domain wall, crosses the beam movement direction 71 in the front portion and in the rear portion of the beam spot. The domain walls are displaced backward from the front portion and forward from the rear portion relative to the heating beam movement direction as shown by the numeral 72 in FIG. 3A. Therefore, the magnetic displacement signals from the front only can be detected by placing record-reproducing beam 73 at the front side of the heating beam-moving direction as shown in FIG. 3B. Similarly, the magnetic displacement signals from the rear only can be detected by placing record-reproducing beam 73 at the rear side of the beam moving directions shown in FIG. 3B.
In either case, the record mark sequence shown in FIG. 2D is reproduced by the record-reproducing beam to give reproduced signals of FIG. 2E, and further giving binary signals shown in FIG. 2F. In the above magneto-optical recording-reproducing method, a light beam is projected to cause displacement of the domain walls of the record marks in the displacement layer by utilizing temperature gradient caused by the light beam without change of the recorded data in the memory layer, and the change of the polarization plane of the reflected light beam is detected to reproduce the record marks. According to this magneto-optical recording-reproducing method, the magnetization states carried by the reproducing beam are the same as shown in FIGS. 3A to 3D. Therefore, the reproduced signals are rectangular, and record marks of less than diffraction limit of the light can be reproduced without decreasing the reproducing signal amplitude. Thereby, a medium for magneto-optical recording and apparatus therefor can be provided which have been improved in recording density and transfer rate.
In FIGS. 3A to 3D, the numeral 77 indicates a switching layer, the numeral 76 indiates a displacement layer, and the numeral 78 indicates a memory layer.
However, the prior art described above has disadvantages of higher cost owing to many optical parts for the heating laser beam, various adjustment steps in assemblage of the apparatus, and two laser system. For solving the problems of the higher cost, the heating and reproduction is required to be conducted with one light beam system.
As shown in FIGS. 5A and 5B, the maximum temperature point in the high temperature region formed by light beam lies in the irradiation range of the light beam. If the heating beam is not employed, the reproduced signal is a synthesized signal formed from two signals: a signal generated by displacement of the domain wall at the front portion of the beam movement direction 71 to the maximum temperature point by temperature gradient, detected at region 81 in FIG. 5A: f(t) (f(t)=0 at t&lt;0), wherein t denotes a reading-out (reproducing) time of the record mark sequence, and when t=0, t denotes a start time of the reading out; and another signal generated by displacement of the domain wall of the rear portion of the beam movement direction to the maximum temperature point by temperature distribution, detected at region 82 in FIG. 5A: .alpha..multidot.3f(t-.beta.) (f(t)=0 at t&lt;0), wherein .alpha. denotes an amplitude gain, and .beta. denotes a delay time; namely the synthesized signal being represented by h(t)=f(t)+.alpha..multidot.f(t-.beta.). The numeral 80 indicates a terminal of the domain wall displacement.
For example, when the recorded signal sequence shown in FIG. 6A is reproduced without the employment of the heating beam, the recorded signals are read out, with the movement of the reproduction beam, through the states shown in FIGS. 6B1 to 6B4 to give reproduced signals as shown in FIG. 6CA, which is superposition of the signals generated by domain wall displacement from the front side (FIG. 6CB) and the signals generated by domain wall displacement from the rear side of the optical beam (FIG. 6CC). Therefore, in this case, the recorded information cannot be reproduced with sufficient margin by conventional technique of binarizing with the slice level of the median of repeated reproduction signals of shortest marks employing the amplitude 79 disadvantageously as shown in FIG. 4.